Lizard nutrition?

Some lizards such as geckos seem to be carnivores; others such as iguanas are herbivores.
I have read on iguana sites that giving iguanas too much/any (depending on who you read) animal protein can be bad for them.
Other lizards such as bearded dragons are commonly fed both insects and vegetables.

Is there actually such a divergence in nutrition? Are there historical reasons, did the lizard line diverge at one time with some evolving as carnivores and others as herbivores?

Hello my dear I take it most of your reptilian experience is with snakes? Not all lizards come from the same lines as an example iguanas and blue belly lizards are iguanids. Tegus and whip tails are examples of teids. Uromastyx and bearded dragons are agamids. Getting back to your examples of iguanas, when they are young in the wild they tend to have more of a proclivity for protein i.e. insects and such because as young growing animals they have a higher need for protein and calcium. As they get older they tend towards less protein in the diet because the need is not as great. You can see it also with red ear sliders as babies they tend to eat more animal prey as adults they incorporate more vegetation in their diet. Also did you know that Monitor lizards and boids call the same ancestor daddy? Uromastyx and bearded dragons are both agamids yet most uromastyx are highly vegetarian where no more than say 5% of their diet includes non vegetation bearded dragons are much more omnivorous. Although even excluseively carnivorous or insectivorous lizards do get vegetation in the gut of the prey they eat.

Digby Rigby

http://exoticfeeders.com the feeder forums

I have heard it is actually bad to give uromastyx lots of insect food. If uros and beardeds arise out of the same ancestry, where is the divergence? What exactly makes it physiologically unsound for the uromastyx to have more than just a bit of insect prey, but allows beardies to have somewhat more? Are their livers/stomachs/digestive systems actually different?
THAT is my question, as it does seem that they have different dietary requirements.

Just because species are from the same taxonomic "FAMILY" does not mean that they are the same physiologically or metabolically. The diet for one may very well be very dissimilar to another.

Example, Iguana iguana, the common green iguana is an obligate herbivore. At some point in time it shared a common ancestor with Anolis ssp. which are insectivores. BOTH are New World reptiles.

Classic Darwinian Speciation can be referred to to explain how these species from the SAME taxonomic FAMILY come to have different dietary needs. The specifics of WHEN the differences occurred is a specialized topic. One would have to research that information by looking into the fossil record for the species.

Generally speaking, dietary differences would be only one of the typical, simple examples of characteristics that separate one species from another

As an aside, all my research and experience has stipulated that the Common Green Iguana NOT be fed any animal protein at any stage of life. I never feed animal based foodstuffs to my iguanas. An unfortunate and common consequence to feeding animal based proteins to juvenile iguanas is that they suffer kidney failure at around 5 to 6 years of age. They can also develop urinary gout due to the excess nitrogenous waste produced from the ingestion of animal based protein. Physiologically, their urinary systems are not equipped to adequately process the high concentrations of nitrogenous waste produced from animal based protein.